1. Field of the Invention
The present invention relates to a switching power supply device for providing a regulated direct current (DC) power supply.
2. Description of the Related Art
With the miniaturization of electronic equipment, it has been more required for switching power supply devices incorporated in electronic apparatus to have a reduced loss, that is, an enhanced efficiency.
Conventionally, as a switching power supply device which can be constructed with a reduced number of the components at a relatively low cost, an RCC (ringing choke converter) as stated in Japanese Unexamined Utility Model Publication No. 63-100993 has been used.
FIG. 26 is a diagram showing an example of a conventional RCC switching power supply device. In FIG. 26, a transformer is designated by T. A switching element Q1 and an input power source E are connected in series with a primary winding T1 of the transformer T. A secondary winding T2 of the transformer T is provided with a rectifying smoothing circuit formed of a rectifying circuit Ds and a smoothing capacitor C.sub.0. A load is connected to the output of the rectifying smoothing circuit. A load is connected to the output of the rectifying smoothing circuit. A detecting circuit 14 detects a load supply voltage. A bias winding T3 of the transformer T is coupled to a controlling circuit 11 for controlling the switching element Q1 in dependence on a 446 voltage produced in the bias winding T3. The controlling circuit 11 causes the switching element Q1 to self-oscillate and control the on-time of the switching element Q1 in dependence on a detected voltage of the detecting circuit 14 through an isolating circuit to regulate the output voltage.
FIG. 27 is a waveform chart at the respective elements shown in FIG. 26. A voltage surge is generated in voltage Vdsl across the switching element Q1 as shown in FIG. 26, since the switching element Q1 is turned off in a state when a current id1 flowing in the switching element Q1 is high. The surge appears in a current flowing in the rectifying element Ds.
It has been pointed out that the conventional RCC switching power supply device has the following problems.
(1) The switching frequency is significantly varied in dependence on an output current. This causes problems such as interference with electronic apparatus and generation of EMI (electromagnetic interference) noises.
(2) The switching loss is large (the switching loss is defined as a product of Vds1 and id1 obtained at turn-on or turn-off of Q1 in FIG. 27).
(3) The energy loss is large, influenced by the leakage inductance of the transformer.
(4) Voltage surge is caused when the switching element is off, as shown in FIG. 27. Accordingly, it is required for the switching element to have a high breakdown voltage.
(5) A voltage surge is caused when the rectifying diode in the secondary is off. Thus, it is necessary that the rectifying diode have high breakdown voltage.
(6) A large reverse recovery loss is caused by the rectifying diode in the secondary circuit.
(7) The EMI noise is large, caused by the switching surge of the switching element and the rectifying diode in the secondary circuit.
In order to solve the EMI noise problem by reducing variation in the switching frequency, especially caused by variation in the output current, a separately-excited fly-back switching power supply device is generally used. The separately-excited fly-back system is characterized in that the switching frequency control and the pulse width control for the switching transistor can be easily carried out.
However, for the separately-excited fly-back system, a separately-excited oscillating circuit needs to be provided for control of the switching transistor. Thus, it is difficult to attain the miniaturization and cost-saving of the device, in contrast to the RCC system.
Japanese Unexamined Patent Publication No. 5-191972 discloses a regeneration control type switching power supply device in which variation in the switching frequency and voltage surge can be inhibited. This regeneration control type switching power supply device has two switching means provided in the circuit thereof. The output voltage is controlled by changing the on/off ratios of the two switching means, and by carrying out the generation action of energy, the variation in the switching frequency, caused by variation in the output current, is inhibited. A regeneration control type switching power supply device of this type is advantageous in that the variation in the switching frequency, caused by the variation in the output voltage, can be inhibited. However, as a result of the generation operation, the change range of a magnetic flux of the transformer is kept substantially constant, irrespective of the output current. This causes problems as follows. In the case where the load is light, the regeneration control type switching power supply device exhibits a larger switching loss at turn-off, as compared with the conventional RCC system switching power supply, since the peak value of the primary current is large immediately before the turn-off. The loss by the transformer is large, since the change range of a magnetic flux in the transformer is maximum. Moreover, since the conduction loss is large, caused by the regeneration current, the power conversion efficiency is low when the load is light. Further, there arises a problem that the conduction loss in the secondary is high, since the peak value of a current flowing in the rectifying element in the secondary is large, and the effective current of the rectifying element in the secondary is high.
For the forgoing reasons, there is a need for a switching power supply device which solves the above-described problems of the RCC system switching frequency power supply device, the separately-excited fly-back system switching power supply device, and the regeneration control type switching power supply device, and which has a high efficiency, low noise, small size and light weight and is produced at a low cost.